1. Field of the Invention
This invention relates to a brazing apparatus having an electrical resistance coil which serves the dual functions of providing radiant heat and acting as an eddy current probe. The invention is particularly useful in brazing Inconel reinforcing sleeves within the heat exchange tubes of nuclear steam generators, where the utilization of the coil as an eddy current probe assists the operator in accurately placing the coil into a proper brazing position along the longitudinal axis of the reinforcing sleeve.
2. Description of the Prior Art
Devices and processes for applying a brazing heat to the inside surfaces of conduits are known in the prior art. Such devices and processes are used to braze reinforcing sleeves within the heat exchange tubes of nuclear steam generators. In such prior art processes, a sleeve which is circumscribed at each end by a ring of brazing alloy is inserted into a tube in need of repair and slidably positioned across the section of the tube where the walls have been damaged due to corrosion or denting. The ends of the tube surrounded by the rings of brazing alloy are then usually hydraulically expanded so that they snugly engage the inner walls of the tube being repaired. To complete the joint, a heater assembly formed from an electrical resistance wire coiled around a mandrel is slid up the sleeve and actuated so that the heat generated by the electrical resistance wire melts the rings of brazing alloy disposed between the outer wall of the sleeve and the inner wall of the tube, thereby forming a pair of watertight braze joints between the ends of the sleeve and the tube. Such prior art sleeving operations are frequently performed in the longitudinal sections of the heat exchange tubes which extend through the tubesheet of the nuclear steam generator due to the tendency of the walls of the tubes to corrode, dent and crack in these regions.
One of the problems associated with the implementation of such brazing processes is the accurate and consistent positioning of the electrical resistance coil adjacent to the longitudinal section of the sleeve which includes the ring of brazing alloy. This problem is particularly acute if one desires to sleeve a region of a tube many feet above the tubesheet, such as a support plate region. If the coil is not in proper position when the heating coil is energized, the heating coil might not succeed in transferring a sufficient amount of heat through the walls of the sleeve to properly fuse the surrounding ring of brazing alloy into the annular gap between the outer walls of the reinforcing sleeve and the inner walls of the surrounding tube. Such an incomplete fusing of the brazing alloy could result in a poor-quality braze joint which does not create the desired, watertight seal between the outer walls of the reinforcing sleeve and the inner walls of the surrounding heat exchange tube. In order to solve this problem, eddy current probes have been developed which are capable of precisely locating the edges of the reinforcing sleeves after the ends of the sleeve have been expanded into engagement against the inner walls of the tube being repaired. An example of such an eddy current probe is found in U.S. patent application Ser. No. 615,868 filed May 31, 1984 by John M. Driggers and assigned to Westinghouse Electric Corporation. When such an eddy current probe locates the edge of the sleeve, the distance between the sleeve edge and the mouth of the tube is noted, and used as a point of reference. Since the longitudinal distances between the edges of the sleeve and the rings of brazing alloy which circumscribe them are known, the operator can easily infer the exact distance which he must slide the radiant heater assembly up through the lontigudinal axis of the tube until the center line of the electrical resistance coil is aligned in the desired relationship with one of the rings of brazing alloy.
While the use of eddy current probes in this manner frequently results in a correct and accurate placement of the coil within the sleeve, this system is not completely free from error. Because the eddy current coils used in connection with prior art brazers cannot survive high temperatures, they must be spaced some distance away from the heating element. Such spacing in turn requires the operator to slidably position the heating element in place after locating the edge of the sleeve by means of the eddy current coils. Misalignments can occur as a result from the compliance of the nylon rod used to slide the heater assembly up to a brazing position within the sleeve. Additionally, if a two-step brazing process is to be used for each braze joint, the center line of the heating coil should be within about 1/8" of the desired position in both steps of the process. In such a two-step process, the coil is used to thermally expand a longitudinal section of the sleeve which is close to, but which does not include, the ring of brazing alloy in order to prevent a corrosion-bound tube from thermally radially expanding during the brazing heat. The advantages associated with the use of such a two-step brazing process in creating high-quality braze joints are specifically set forth in U.S. patent application Ser. No. 634,336, filed July 13, 1984 now U.S. Pat. No. 4,620,662 by John M. Driggers and assigned to Westinghouse Electric Corporation, the entire specification of which is hereby expressly incorporated herein by reference.
Clearly, there is a need for some means for correctly and reliably positioning the electrical resistance coil of a radiant heater assembly within the reinforcing sleeves used to repair damaged heat exchange tubes in nuclear steam generators, in order to consistently produce high-quality, watertight braze joints between the sleeve and the tube. Ideally, such means should be able to withstand the brazing heat generated by the coil so that the distance between the heating coil and the eddy current sensor can be minimized, and should not be prone to the inaccuracies which result from the compliance of the flexible rods used to slide the heater assembly up and down the longitudinal axis of the sleeve/tube combination.